This invention relates to the manufacture of internally grooved tubes and, more particularly, to an improved method of cold drawing tubes for forming continuous shallow grooves, narrowly spaced apart in either an axial or spiral orientation, on the inside surface of the tubes, and an apparatus therefor.
Known methods have been utilized to place grooves on the internal surfaces of tubes for different purposes. Such methods include machining, broaching, informing, extruding and drawing techniques.
Various grooving techniques are described in patent disclosures.
Hackett (U.S. Pat. No. 2,392,797), for example, discloses a technique for imparting rifling, fluting, ridging or the like to an internal tubular surface, particularly for a gun barrel or liner, through the use of a die and a mandrel arrangement including a mandrel having a surface configuration which is converse to that to be imparted to the tube. The die compresses the tube onto the mandrel, by relative axial movement of the tube and the die, as the tube moves through the die.
In Harvey, et al (U.S. Pat. No. 2,852,835), an apparatus is disclosed wherein metallic tubing is drawn through an annular formed by a stationary die and a cooperating rotatable rifling mandrel for simultaneously sizing the tubing and forming spiral projections on the interior surface of the tubing. The die includes a tapered frusto-conical lead-in portion followed by a cylindrical portion which gradually reduces the outside diameter of the tube to the desired final outside diameter. The initial contact of the internal surface of the tube on a portion of the rifling mandrel and the contact of the outer surface of the tube with the tapered lead-in portion of die occur concurrently. Hence, the spaced portions of the inside surface of the tube are radially forced into the grooves of the rifling mandrel simultaneously with a portion of the outer surface diameter reduction. No specific type of groove geometry is disclosed although the patent indicates that the technique is useful for the production of rifled aluminum barrels and the like.
Drawing techniques similar to that of Harvey, et al (U.S. Pat. No. 2,852,835) are shown by Nakamura, et al (U.S. Pat. No. 3,830,087), Koch, et al (U.S. Pat. Nos. 3,289,451 and 3,088,494), Hill (U.S. Pat. No. 3,292,408), House (U.S. Pat. No. 3,487,673), Sirois (U.S. Pat. No. 3,744,290), Stump (U.S. Pat. No. 4,161,112), and Tatsumi (U.S. Pat. No. 4,373,366). Grover, (U.S. Pat. No. 3,865,184) and Runyan, et al (U.S. Pat. No. 3,753,364), for example, both teach a horizontally disposed heat pipe as well as a method and apparatus for fabricating the heat pipe. Grover (U.S. Pat. No. 3,865,184) is primarily directed towards the actual heat pipe apparatus itself, describing, in detail, the very particular structure desired. Runyan, et al (U.S. Pat. No. 3,753,364) is primarily directed to a method and apparatus for producing capillary grooves on the inside tube surface of the heat pipe. The disclosed method and apparatus provide a means for fabricating a spiraled capillary groove by cutting the metal from the wall of the tube and raising and folding the cut metal over to provide a groove having a narrow opening for maximum capillary action. The cutting tool has a curved planar edge formed by the intersection of a planar surface and a cylindrical surface. The grooves produced thereby may have dimensions of a peak to trough depth on the order to 0.014 inches (0.3556 mm) and a spacing on the order of 0.007 inches (0.1778 mm) with the opening of the grooves narrower than the width of the grooves to provide optimum capillary action. The use of separate annular grooves of the same geometry is also disclosed. The method of placing the grooves in this inner tube wall surface is one of cutting with a cutting tool, and not a cold-drawing process.
When the metal for the inner surface of a tube shell is forced radially into grooves of a mandrel, there is a tendency for the metal to elongate along the longitudinal direction of the groove rather than radially filling the groove. This problem is exasperated as groove depth increases, as spacing between the grooves decreases, as drawing speed increases and, as well, in the case of hard metal workpieces.
In practice, no cold drawing method is known to the inventor which has been succesfully demonstrated as capable of making continuous shallow grooves in a hard metal such as steel, for example, continuous grooves having a depth of 0.020 inches (0.508 mm) with 0.040 inches (1.016 mm) between the grooves. More particularly, no cold drawing method is known to the inventor which is capable of rapidly making, in hard material, shallow continuous grooves that exhibit a uniform spiral along the length of the tube. Such grooves have particular application to heat pipes which use capillary grooves to transfer condensate from a condenser to an evaporator as the tubes exhibit increased heat transfer due to the extended surface and, accordingly, would be optimum "wicks" when used in thermosyphon-type heat pipe applications.